Santosh Biswas (27) of Chapra Maharajpur was allegedly killed by Shankar Biswas on Friday night.
A complaint was lodged with Chapra police.
Police said, "We have taken Shankar Biswas in custody and are interrogating him."
According to local people, Shankar was a construction labourer and used to work in other states. He returned home a few months back.
He had protested against the illicit relationship of his mother and Shankar.
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